1. Field of the Invention
The present invention relates to a power supply unit having a dimmer function for driving a semiconductor light emitting module so as to light the semiconductor light emitting module at suitably dimmed brightness, and a lighting unit having this power supply unit.
2. Description of the Related Art
Recently, from a viewpoint of energy saving, semiconductor light emitting modules such as light-emitting diodes are used as light sources for lighting units, and DC power supply units into which switching elements are incorporated are developed as power supplies which drive the semiconductor light emitting modules such as the light-emitting diodes. As these power supply units, it is known that they have a dimmer function for adjusting brightness of the light-emitting diodes according to a dimmer signal given from the outside.
Conventionally, the power supply unit having such a dimmer function is disclosed in, for example, JP-A 2003-157986 (KOKAI). The power supply unit disclosed in this publication has a voltage dimmer circuit which controls an applied voltage to light-emitting diodes, and a duty dimmer circuit which switching-controls an applied voltage to the light-emitting diodes. The voltage dimmer circuit and the duty dimmer circuit are changed over to be controlled according to a dimmer control signal.
In the power supply unit disclosed in JP-A 2003-157986 (KOKAI), a DC voltage given to the light-emitting diodes is adjusted according to a pulse width of the dimmer signal, and an applied voltage to the light-emitting diodes is switched so that the light-emitting diodes are controlled to be dimmed. Therefore, output light from the light-emitting diodes has a problem that flicker easily occurs. In addition to a current limiting function for controlling an output current according to the pulse width of the dimmer signal, a switching element which is in series or in parallel with the light-emitting diodes is necessary, and thus the number of parts increases and circuit efficiency is deteriorated. Since the pulse width is controlled, when a switching frequency for this control is in an audible area, a noise might be generated.
On the other hand, since the light-emitting diodes have an approximately constant voltage characteristic, a part or a device having the current limiting element is necessary for stable lighting. In order to control an electric power in a power supply unit using a switching element, current control is generally used. In the current control, an element temperature of the light-emitting diodes is determined by a value of an electric current flowing in the light-emitting diodes, and the element temperature influences an element life. Therefore, in the current control, the flowing electric current is the important control element due to design of the lighting unit.
The dimming of the light-emitting diodes can be realized comparatively more easily than a discharge lamp lighting unit. The light-emitting diodes as a load have stable electric characteristics, and fluctuation in the brightness of the light-emitting diodes due to an external factor such as temperature is small. For this reason, the dimming of the light-emitting diodes can be easily realized. In the application of deep dimmer control, namely, brightness control where a brightness control rate or a dimmer rate is set large and thus brightness of the light-emitting diodes is greatly reduced, the constant current control is adopted to the light-emitting diodes. In this constant current control system, the light-emitting diodes can be lighted stably in a control area where lighting current is high for full-emission lighting. In this system, however, the lighting current supplied to the light-emitting diodes is lowered in the deep dimmer control area, and a current detecting signal becomes minute according to the lowering of the lighting current, and a reference current for controlling the lighting current is a minute signal. Therefore, in a constant current control circuit, accuracy of a detecting circuit or a comparator requires high performance, and the control circuit is easily influenced by a noise, so that a stable operation becomes difficult. It is thus considered that a signal voltage for control is increased. However, the current detecting signal is generally detected by a resistor inserted in series into the light-emitting diodes, and resistance of the resistor should be increased in order to increase the detecting signal. As a result, in the control area where the electric current flowing in the light-emitting diodes is high, the electric power is greatly consumed by the detection resistor, or heat is generated from the detection resistor, and a countermeasure against this heat inhibits developments of products.
As a control system which solves these problems, a constant voltage control system which constantly controls an output voltage is also proposed. A voltage for turning on the light-emitting diodes is higher than that for a general. silicon diode. For example in a GaN type diode represented by blue one, an electric current starts to flow at about 2.5 V, and about 3.5 to 4.5 V in the full-emission lighting, and the brightness of the light-emitting diodes can be controlled comparatively stably without being influenced by the performance of the light-emitting diodes or a noise generated on the light-emitting diodes even in the deep dimmer control. However, a forward voltage of the light-emitting diodes has a negative temperature characteristic, and the forward voltage is decreased due to self heat generation at the time of applying an electric current to the light-emitting diodes, and the electric current increases. As a result, heat generation becomes large, and thus thermo-runaway might occur. The forward voltage of the light-emitting diodes greatly varies, and even if an output from the lighting unit is adjusted, output currents vary due to a individual difference of respective light-emitting diodes.
The above-mentioned problem arises not only in the semiconductor light emitting modules such as the light-emitting diodes but also in the power supply units which light a light source such as an organic EL light source or an inorganic EL light source developed in recent years, and this problem still remains unsolved.
The power supply unit and the lighting unit having the power supply unit which can realize the stable dimmer control are already proposed as a prior application in International Application No. PCT/JP2009/055871 filed on Mar. 24, 2009 by the same assignee. In the power supply unit of the International Application, first and second reference signals, which change according to a dimmer rate of a dimmer signal, namely, a dimmer level, are prepared. In the almost full-emission lighting control area where the dimmer rate is small, the first reference signal is selected, and light-emitting diodes are controlled with constant current with reference to the first signal. In a lighting control area where the dimmer rate is large and the brightness is reduced, the second reference signal is selected, and the light-emitting diodes are controlled with constant voltage with reference to the second reference signal. Since the first and second reference signals are selected so that the light emission from the light-emitting diodes is controlled, the stable dimmer control can be realized.